The present invention is directed to improved blower housings of the type used to surround fans. More specifically, the present invention contemplates a blower housing with a cutoff having a radial dimension relative to the fan axis where the radial dimension varies from a greater distance at a cutoff midsection to a lesser distance at the cutoff ends. Moreover, the thickness of the cutoff face varies from narrower ends to a wider midsection, the cutoff angle varies from end to midsection, and the slope of the cutoff face may vary.
Previous blowers, such as that shown in U.S. Pat. No. 5,279,515 to Moore et al., include a scroll housing which expands from a cutoff in a continuous and smoothly increasing radial dimension from that cutoff to a discharge outlet. The scroll housing is enclosed by a pair of side walls to enclose a blower and to form a discharge plenum. The discharge plenum is outside of the blower's periphery and inside of the scroll housing and sidewalls. The plenum is characterized by a continuously increasing cross-sectional area basically formed by the radial expansion of the scroll housing away from the periphery. This discharge plenum is defined by a rectangular footprint in a plane perpendicular to the axis of the blower and having edges tangent to the scroll housing at locations spaced approximately 90° from each other. The cutoff is linear and parallel to the axis of the rotation of the fan.
U.S. Pat. No. 5,570,996 to Smiley, III shows a scroll housing having a conformal portion of constant radius preceding the expansion portion of the scroll housing.
U.S. Pat. No. 5,868,551 to Smiley III et al. shows a cutoff for a tangential fan. The fan cutoff 120 has an edge 122 proximal the tangential fan where the edge is not parallel to the fan axis but instead is skewed relative to the axis 14 so that the edge spirals around the periphery of the tangential fan preferably while maintaining a constant gap G between the fan 12 and the edge 122. Effectively, the cutoff angle changes but the gap does not.
U.S. Pat. No. 5,772,399 to Mehta et al. shows a centrifugal fan 10 using a cutoff faring 32. Being of slideable construction, the cutoff faring 32 may be extended a greater or lesser distance into the exit port 15. This is illustrated by a comparison of FIGS. 5 and 6 where the cutoff faring is extended the fullest possible distance H1 in FIG. 5 as opposed to the lowest distance H2 in FIG. 6. The cutoff is linear and parallel to the axis of rotation of the fan.
U.S. Pat. No. 6,677,564 to Shon et al. shows a microwave oven having a blower apparatus with a cutoff portion. The shape of the cutoff portion forms a ‘V’ shape or a ‘U’ shape, and a first inclined surface 471 and a second inclined surface 472 can be formed as a straight or curved line.
Cutoffs are a tradeoff between efficiency preventing recirculation of air from the discharge path, stability of fan operation, and quietness of the fan. Previous cutoffs such as described above are usually a compromise between efficiency, stability, and sound levels but not all three. It would be desirous to provide a cutoff for a fan or blower which is both highly stable and efficient in its operation and having an optimum sound level.